When enabling A20, put upper limit on amount of time we wait for the
keyboard controller to get ready (65K x ISA access time, visually
around 1 second). If we have wait more than that amount it's likely
that the hardware is a legacy-free one and simply doesn't have keyboard
controller and doesn't require enabling A20 at all.
Taken-from: FreeBSD (with modifications suggested by Matt)
relavant commit logs(and discussion):
http://docs.FreeBSD.org/cgi/mid.cgi?
200604110439.k3B4dTOD072774
http://docs.FreeBSD.org/cgi/mid.cgi?
200604112053.k3BKrnC1053262
http://docs.FreeBSD.org/cgi/mid.cgi?
200604260605.k3Q65GZZ095546